今天來講講前幾次也有出現的 LiveData，以下如有解釋不清或是描述錯誤的地方還請大家多多指教：

什麼？

LiveData 是一個具有生命週期感知的 observable data holder class，只在依附的 LifecycleOwner 是 active 的狀態下才會進行 observer，只有在資料狀態改變時才會通知觀察者，可透過以下兩種方式來更新 LiveData。

• MainThread 時：setValue(T)
• WorkerThread 時：postValue(T)

| Transform LiveData

以下分成 map 和 switchMap，這兩者的差別在於一個回傳物件，一個要回傳 LiveData 物件，那這會用在什麼地方呢，當你需要將監聽的 LiveData 值再轉換成另一個樣子動態通知另一個 view 去做變化：

• Transform.map

val cityLiveData: LiveData<CityCard> = CityLiveData()
val cityName: LiveData<String> = Transformations.map(cityLiveData) {
    city -> "${city.name}"
}

• Transform.switchMap

val cityLiveData: LiveData<CityCard> = CityLiveData()
val cityName: LiveData<String> = Transformations.switchMap(cityLiveData) { city -> 
		liveData {
        emit(city.cityName)
    }
}

| MediatorLiveData

合併多個 LiveData，只要其中一個有變化就會發通知出去，使用情境在於，假如今天你的介面的資料來源有很多個，這時就可以使用 MediatorLiveData 去監聽多個來源的資料變化：

val cityList = MediatorLiveData<List<CityCard>>()

fun setData() {
	cityList.addSource(firstLiveData) { value ->
        cityList.value = value
    }
    cityList.addSource(secondLiveData) { value ->
        cityList.value = value
    }
}

如何？

library 的部分依樣在 lifecycle 那邊就已經加好了，直接來設置 LiveData，這邊會使用 private 的 mutableLiveData 來進行 setValue，由外部監聽 LiveData，確保只能由 ViewModel 來變動值，外部只能取得值不能改變值，並包覆之前所寫的 Resource：

// 新增城市
private val _card = MutableLiveData<Resource<CityCard>>()
val card: LiveData<Resource<CityCard>> = _card

// 取得 db 城市列表
private val _cardList = MutableLiveData<Resource<List<CityCard>>>()
val cardList: LiveData<Resource<List<CityCard>>> = _cardList

透過 Resource 傳達出去的狀態，可以顯示資料讀取狀態的 layout

class MainViewModel: ViewModel() {
    private val _isCardInserted = MutableLiveData<Resource<Boolean>>()
    val isCardInserted: LiveData<Resource<Boolean>> = _isCardInserted

    private val _cardList = MutableLiveData<Resource<List<CityCard>>>()
    val cardList: LiveData<Resource<List<CityCard>>> = _cardList

    fun getForecast(country: String) {
        _isCardInserted.value = Resource.Loading()
        val service = WeathbyRetrofit.makeRetrofitService()
        viewModelScope.launch {
            runCatching {
                service.getForecast(query = country)
            }.onSuccess {
                it.body()?.apply {
                   setCardDB(CityEntities(...))
                }
            }.onFailure {
                _isCardInserted.value = Resource.Error(it.message ?: "連線逾時請稍後再試")
            }
        }
    }

    private fun setCardDB(city: CityEntities) {
				val db: CityDAO = CityDatabase.getInstance(WeathApplication.instance.applicationContext).cityDao() 
        viewModelScope.launch {
            runCatching {
                db.insertAll(city)
            }.onSuccess {
                _isCardInserted.value = Resource.Success(true)
            }.onFailure {
                _isCardInserted.value = Resource.Error(it.message ?: "連線逾時請稍後再試")
            }
        }
    }

    fun getCardDB(db: CityDAO) {
        _cardList.value = Resource.Loading()
				val db: CityDAO = CityDatabase.getInstance(WeathApplication.instance.applicationContext).cityDao() 
        viewModelScope.launch {
            runCatching {
                db.getAll()
            }.onSuccess {
                _cardList.value = Resource.Success(
                    it.map { 
                        CityCard(
                            ...
                        )
                    }
                )
            }.onFailure {
                _cardList.value = Resource.Error(it.message ?: "連線逾時請稍後再試")
            }
        }
    }
}

View 可以這樣監聽：

private fun setupViewModel() {
    viewModel.isCardInserted.observe(viewLifecycleOwner) {
        when(it) {
            is Resource.Loading -> {
                showLoading()
            }
            is Resource.Success -> {
                hideLoading()
                if (it.data == true) viewModel.getCardDB()
            }
            is Resource.Error -> {
                hideLoading()
            }
        }
    }

    viewModel.cardList.observe(viewLifecycleOwner) {
        when(it) {
            is Resource.Loading -> {
                showLoading()
            }
            is Resource.Success -> {
                hideLoading()
                it.data?.apply {
                    cardAdapter.submitList(this)
                }
            }
            is Resource.Error -> {
                hideLoading()
            }
        }
    }
}

之後 db 會以注入的方式取得

Reference

Android LiveData